Mass spectrometer electron beam ion source having means for focusing the electron beam



Dec. 5, 1 967 P E. M ELLIGOTT 3,355,343 MASS SPEUTROMETER ELECTRON BEAM ION SOURCE HAVING MEANS FOR FOCUSING THE ELECTRON BEAM Filed Feb.

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Dec. 5, 1967 'P. E, M ELLIGOTT MASS SPECTROMETER ELECTRON BEAM ION SOURCE HAVING .MEANS FOR FOCUSING THE ELECTRON BEAM 2 Sheets-$heet 2:

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m m w mm m M ma Z W mm I WW I m w ma m w w o a C mh U CC \5 a 9 4 4 5 6 v 4 /n vnfor; Pefer 5 Me E/l/goh F/e/d Strength Fie/d Sire/1 W? His Afro/nay United States Patent 3,356,843 MASS SPECTROMETER ELECTRON BEAM ION SOURCE HAVING MEANS FOR FOCUSING THE ELECTRON BEAM Peter E. McElligott, Schenectady, N.Y., assignor to General Electric Company a corporation of New York Filed Feb. 1, 1965, Ser. No. 429,325 1 Claim. (Cl. 250-419) ABSTRACT OF THE DISCLOSURE Electrostatic focusing means for the electron source of an ionizing device, means for the control of the focusing means and means for automatically maintaining the electron flow substantially constant are described.

The electrostatic focusing means includes an inner and an outer spaced concentrically-mounted electrically conducting cylindrically-shaped element, each of which elements has a straight longitudinal slit through the wall thereof, the elements being located with the filament mounted along the centerline of the inner element with both slits in register with each other and with the filament.

This invention is directed to ionization sources for apparatus such as a mass spectrometer and more particularly to an improved configuration for an ionization source wherein ions are produced and delivered into the accelerating field of a mass spectrometer With a minimum spread in the ion energies whereby the ions can then be separated according to their m/ e (mass/ charge) ratio with greater resolution.

A mass spectrometer is any of a wide assortment of apparatuses, which are capable of sorting streams of electrified particles known as ions in accordance with their different masses by means of deflecting fields. Because of this capacity, mass spectrometers are very useful analytical tools. Thus, for example, by the use of such a device the gas density in an evacuated system may be determined. As an example, a partial pressure gage is actually'a mass spectrometer. In a partial pressure gage a portion'of the gas present in a given system is converted into ions and the relative numbers of ions of each mass, or mass/charge (m/e), present are measured by passing the ions into an analyzing region that separates the ions according to their m/e. ratio, and then to a detector that measures the number of ions of a particular m/ e ratio.

Ionization is usually accomplished by bombardment with electrons from a hot filament, although, cold cathode, spark sources, field ionization, and other sources have been used. Detectorsare either Faraday cage collectors or ion multipliers. The latter detector is generally used atvery low pressures, i.e., low ion concentrations. The types of m/e analyzers vary greatly. Herein the embodiment described by way of example employs magnetic deflection analysis.

To be of maximum utility in the performance of a mass spectrometer it is preferable that the ions delivered to the detector in the mass spectrometer have energies of approximately equal magnitude and the smaller the energy spread among the ions delivered, the greater the resolution of the spectrometer. It is, therefore, a prime, object of this invention to provide an improved ionization source for a mass spectrometer having means for focusing the ionizing electron beam on a specific volume of the ionizing chamber.

It is another object of this invention to provide an improved ionization source for a mass spectrometer wherein means are provided for applying a repelling force to aid 3,356,843 Patented Dec. 5, 1967 in the extraction from the ionization chamber to the analyzing region of the mass spectrometer of the ions generated in the ionization chamber.

It is a further object of this invention to provide in combination with an ionization source means for collecting emitted electrons which have passed the target zone and can thereby serve to generate a collector current, which may then be used to regulate and monitor the power input to the filament in order to maintain constant the flow of ionizing electrons through the ionization chamber.

These and other objects and advantages are provided in an improved ionization source wherein an electrostatic lens system composed of two concentric cylindrical segments surrounding a long filament focuses the ionizing electron beam on a specific volume of the ionization chamber; means are provided for applying a repelling force to the ions generated in the ionization chamber to aid in conducting the ions to the mass spectrometer, and means are provided to regulate and monitor the current responsible for generating the electron beam.

The exact nature of this invention as well as other objects and advantages thereof will be readily apparent from consideration of the following specification relating to the annexed drawings in which:

FIG. 1 is a schematic representation of a prior art construction of an ion source shown as part of a mass spectrometer;

FIG. 2 is an elevational view of an ion source shown partially in section constructed in accordance with this invention with the ion source forming part of a mass spectrometer, the balance of which is shown schematical- FIG. 3 is a plan view of the ion source shown in FIG. 2 as taken on line 33;

FIG. 4 is a circuit diagram showing the manner in which power is supplied to the ion source of this invention and also the means for monitoring and regulating power supplied thereto; and

FIG. 5 is a graphic display showing a type of error in resolution diminished with the construction of this invention.

The equipment schematically illustrated in FIG. 1 is a mass spectrometer 10 employed as a partial pressure gage and equipped with a conventional ion source 11 resembling an overturned metal mesh basket, cage 12, which is supported on base plate 13. Filament 1-4 is a simple wire coil extending along one side of cage 12 and supplied with current from power source 16. Cage 12 is maintained at a positive potential relative to filament 14 in order to encourage the passage of electrons boiledoif filament 14 to and into cage 12 to bombard gas molecules that are present therein. Ion source 11 is mounted in a housing 17 in communication with the vacuum system in order to be exposed to the conditions therein. Ions created by the bombardment of gas molecules within the ion cage 12 by electrons from filament 14 exit through opening 18 under the attracting influence of a drawout potential applied in zone 19 of spectrometer 10. In order to effectively attract ions from cage 1-2, the drawout potential applied between drawout plate 20 and base plate 13 must be large i.e. as much as a thousand volts or more in the operation of this conventional arrangement. Focusing electrodes 21, '22 serve to center in analyzer tube 23 the beam of ions passing from zone 19 to analyzer tube 23 under an accelerating force. Here, in

analyzer tube 20 the ions are separated according to m/e (passing perpendicular to the plane of FIG. 1), they experience an acceleration and are given circular trajecto- J ries. The ions then pass to ion multiplier 26 wherein the number of ions for each m/ e is detected.

A preferred embodiment of the novel construction of this invention is shown in FIGS. 2 and 3 as part of the partial pressure gage 30 wherein ionization source 31 comprises a combination of several elements arranged on base plate 32 by which ionization source 31 is supported in position, as for example, in the mass spectrometer 30. The ionizing electron beam required for generation of ions within ionization chamber, or cage 33 emanates from a long wire or ribbon filament 34 located Within and along the center axis of a pair of concentrically mounted cylindrical segments 35 and 36, which comprise an electrostatic lens system. In a typical installation cage 33 would be made of gold-plated molybdenum with each of two opposing ends thereof consisting of an open-grid arrangement (one such end is shown in FIG. 2). The other two opposed sides and the top of cage 33 (shown in cross-section) are of the solid metal with appropriate openings therein as for windows 33a, 33b, each of which is a longitudinally extending slot covered with a grid to secure equal potential over substantially the entire open area of windows 33a and 33b. Other metals may be employed for the solid and mesh portions of cage 33, the criteria being that the metal be electrically conducting and not readily oxidized. Electrostatic lenses 35 and 36 in a typical installation Would be constructed of the same metal employed for cage 33. Preferably filament 34 is made of iridium coated with lanthanum boride. The lanthanum boride emits electrons easily for this material has a relatively low work function, also, this material is not easily poisoned.

The electrostatic lenses 35 and 36 mounted in annular recesses in end plates 37, 38 made of an insulating material such as alumina are used to focus the electron beam, which is produced by passing a sufiiciently high current through filament 34, so that the electron beam is concentrated in a specific volume or zone in the ionization chamber 33. This focusing action is effected by adjusting the relative potentials of lenses 35 and 36 as will be more completely described below.

As a result of this facility for concentrating the electron beam, the ions which are generated within ion cage 33 have substantially the same energy level in contrast to the spread in ion energies, which occurs in the prior art ion cage 12. This minimizing of the energy spread among the ions generated by the bombarding of gas molecules substantially increases the resolution of mass spectrometer 30 as will be more completely described in connection with FIG. 5. Also, by using a long straight configuration for filament 34 high emission currents may be obtained therefrom, an added advantage.

Outer lens 36, in addition to its use as a focusing element, may also be used to create a potential barrier for electrons emitted by filament 34. Thus, by adjusting the potential of lens 36 so that it has a negative charge, electrons with insufiicient energy to overcome the repelling action of lens 36 cannot enter the ionization chamber 33.

Ions generated in ion cage 33 leave the ion cage through opening 39 leading to the accelerating zone 41 of mass spectrometer 30. In addition to attracting the ions from the interior of cage 33 by means of the penetrating drawout potential applied between drawout plate 42 and base plate 32, this invention also employs means for repelling the ions out of cage 33 and into accelerating field 41 beyond. This repelling means is shown in FIGS. 2, 3, and 4 as repeller plate 43 with the repelling force being provided by maintaining plate 43 at a positive potential of 1 to 2 volts relative to the ionization cage potential. By this mechanism smaller voltages can be applied for the drawout potential and more nearly uniform field gradients will result in ion cage 33 than in the case in which reliance is placed solely on the use of drawout potentials. In this way the spread in the energies of ions produced in cage 33 is further minimized.

On the opposite side of ion cage housing 33 from the filament 34 is positioned electron collector cup 44. This collector cup 44 is maintained at a positive potential with respect to the ion cage 33 in order to prevent the secondary emission electrons, which are produced on impact therein of the ionizing electron beam from moving to and into the ion cage 33. By preventing entry of the secondary emission electrons into the ionization region, the eifectiveness of ion generation having a reduced spread in ion energies is further promoted.

In addition, it becomes possible to use the collector current (rather than the electron emission current as is the practice in prior art devices) in order to monitor and regulate the power input to the filament 34. Circuitry for such regulation is shown schematically in FIG. 4. The advantage of this mode of regulation is that operation of the ion source is further stabilized, because the flow of ionizing electrons through the ion cage 33 is maintained constant rather than maintaining constant the total electron emission from the filament 34.

Monitoring of the power input to filament 34 by the use of the current generated at collector cup 44 is effected Within constant current emission regulator 45. First, a reference current is selected by the operator and variable potentiometer 46 is set to provide this reference current to the comparator circuit 47. The output from collector cup 44 is connected in series with the comparator circuit 47 and as collector current is generated during operation of the ion source 31, this current passes to the comparator circuit 47 and is compared to the reference current therein. The output of comparator circuit 47 which represents any difference (error current) between the values of these currents, is connected in series with the constant current charging circuit 48. Constant current charging circuit (CCCC) 48, unijunction transistor (UJT) firing circuit 49 and the silicon controlled rectifier (SCR) circuit 50 are connected in series as shown and the output of SCR circuit 50 is connected to transformer 52, which thereby impresses a voltage upon filament 34.

If the error current is very large, as when filament 34 is too cold and not enough electrons are emitted, such that the collector current is much smaller than the reference current, the CCCC 48 reaches its threshold voltage early in each half of any given alternating current cycle. The value of the threshold voltage is set by the UJT firing circuit 49 and, when this threshold voltage is exceeded, the UJT firing circuit 49 fires. When this firing is caused to occur, the SCR circuit 50 fires. Thus, if the threshold voltage is exceeded early in each half of any given alternating current cycle, the UJT firing circuit 49 and the SCR circuit 50 are caused to fire in sequence early in each half of the cycle with the SCR circuit 50 supplying a surge of voltage to the filament 34. The earlier in the cycle the firing occurs, the greater the surge of voltage from SCR circuit 51 to filament 34.

The maximum voltage which can be supplied in this manner is limited by the predetermined setting of a variable potentiometer (not shown) in SCR crrcult 5i. The greater the impressed voltage, the more current is supplied to filament 34 resulting in greater electron emission therefrom as the filament grows hotter. This in turn results in an increase in the collector current and a decrease in the error current, thereby enabling self-adjustment of the system in accordance with the setting of potentiometer 46.

As the collector current approaches the reference current in value, firing of the CCCC 48, the UJT firing circuit 49 and the SCR circuit 51 in sequence during each half of any given alternating current cycle is retarded and the voltage supplied to the filament is gradually reduced. If conditions are such that the error current exceeds the reference current, the firing sequence is so greatly retarded as to supply very little voltage to the filament 34, thereby resulting in a decrease in the collector current.

As shown in FIG. 4, variable potentiometers 53 and 54 enable adjustment of the potentials applied to electrostatic lenses 35 and 36, respectively. During adjustment of the potential of lenses 35 and 36, the ideal relationship is determined when the ratio of the collector current to the cage current is at a maximum. Thus, by detecting the relative values of the collector current (ammeter 57) and the cage current (ammeter 56) the setting of the lens potentials may be facilitated.

As an illustration of the mode of focusing of this invention, certain parameters are fixed; i.e., the ionization potential (potentiometer 58) may be set at about 75 volts, the repeller potential (potentiometer 59) may be set at -2 volts and the accelerating potential, which is the difference in potential between base plate 32 and analyzer 63 (at ground potential), may be set at 500-1500 volts. Next the focusing electrodes 61, 62 are set to center the ion beam in analyzer portion 63 and the drawout potential between drawout plate 42 and base plate 32) is set at some value not in excess of 60% of the acceleration potential. This latter setting represents a 2050% reduction over the value required in conventional construction. Focusing of lenses 35, 36 is then accomplished by setting the reference current in the comparator circuit 47 by means of potentiometer 46 to register a collector current on ammeter 57 of about 30 microamperes and then adjusting the voltages of lenses 35, 36 to yield a minimum cage current (ammeter 56) relative to the collecter current. The potential of either of lenses 35, 36 may be set first and the potential of the other varied accordingly.

It should be noted that this combination of lenses 35, 36 is operative with the potential of lens 35 being set at some value between a slight negative value and a positive value. With the ratio of collector current cage current at a maximum the optimum focusing condition has been reached and the control device illustrated in FIG. 4 serves to maintain optimum operating conditions.

The mass spectrometer 30 is then operated in the conventional manner, that is, the number of ions of each particular mass/ charge ratio (m/e) is determined. By reference to the following equation:

(wherein K is a constant whose value depends upon the value of the radius of analyzer 63, r is the value of this radius, H is the magnetic field strength in gauss and V is the value of accelerating voltage in volts) The operator can selectively vary either the accelerating voltage or the magnetic field strength (or both) in order to sweep over selected portions of the mass spectrum. The ion currents, which are recorded in the ion multiplier 64 at particular values of m/ e, are amplified and displayed on a recorder, or oscilloscope (not shown), providing traces similar to those shown in FIG. 5. Such traces represent a sweep over a portion of the mass spectrum. As shown, ion current as a function of field strength is displayed and field strength is correlated to m/e by means of Equation 1.

In order to illustrate the improvement achieved by the use of this invention FIG. 5 provides a comparison of the traces that would be produced covering the same portion of the mass spectrum in detecting partial pressure in the same system. The prior art trace (shown on the left), because of its poorer resolution, causes an overlap between the trace indications of M and M (region 71). This prob lem is particularly severe, when one mass, i.e. M is present in considerably greater concentration (greater amplitude) than the other. In such a case the aforementioned overlap will cause a higher (erroneous) reading for the concentration of M than actually exists. By eliminating such overlap (as in the right hand trace) this error can be greatly reduced and such elimination is accomplished by the use of this invention wherein a much more narrow band of ion energies is presented to the analyzing portion of the spectrometer.

It should be understood, of course, that the foregoing disclosure relates to only a preferred embodiment of the invention and that modifications or alterations may be made therein without departing from the spirit and scope of the invention as set forth in the appended claim.

What I claim as new and desire to secure by Letters Patent of the United States is:

In an ionizing device wherein electrons are liberated from a straight longitudinally-extending filament to generate an ionizing electron beam by supplying electrical power to said filament, which electron beam is directed by focusing means into an adjacent enclosure to produce ionization of molecules contained therewithin, the improved focusing means comprising:

(a) an inner and an outer spaced concentricallymounted electrically conducting cylindrically-shaped element,

(1) said elements each having a straight longitudinal slit through the wall thereof and being located with the filament mounted along the central axis of the smaller diameter inner element and having both slits of said elements facing the enclosure in register with each other and with said filament,

(b) first adjustable means connected to said inner element for applying electrical potential thereto to exert an electrostatic force on said filament and (c) second adjustable means connected to said outer element for applying electrical potential thereto to exert an electrostatic force on said filament.

References Cited UNITED STATES PATENTS 2,544,716 3/1951 Nier 250-41.9

2,611,875 9/1952 Washburn 250-4l.9

2,792,500 5/1957 Burk 25041.9

2,813,978 11/1957 Brenholdt 25041.9 2,858,464 10/1958 Roberts 31382.1

2,930,917 3/1960 Nief 25041.9

RALPH G. NILSON, Primary Examiner.

A. L. BIRCH, Assistant Examiner. 

